U.S. Pat. No. 7,738,750 assigned to the assignee of this disclosure and incorporated herein by reference discloses various embodiments of a compact, low-cost optical wavelength multiplexer/demultiplexer. Such embodiments will be referred to herein as multiplexers since they perform optical wavelength multiplexing or optical wavelength demultiplexing depending on the direction in which light travels through them. Accordingly, as used in this disclosure, the term multiplexer encompasses a multiplexer and a demultiplexer, the exact function depending on the direction in which the light travels. Similarly, the term multiplexing encompasses multiplexing and demultiplexing, depending on the direction in which the light travels.
The above-mentioned multiplexers output two or more light beams incident thereon at respective, different locations as at least one combined light beam that will be referred to herein as an output beam. In a typical application, the multiplexer outputs the output beam into an output path that conveys the output beam to a destination optical element. The output path typically comprises an optical fiber or other type of optical waveguide and is characterized by an acceptance range. An output beam incident on the output path within the acceptance range of the output path enters the output path and is conveyed by the output path to the destination optical element. The acceptance range of the output path has a spatial component and an angular component that interact such that the spatial component is at a maximum when the angular component is at a minimum and vice versa. In other embodiments, the output path comprises a free-space link to the destination optical element. In this case, the destination optical element has an acceptance range that defines the acceptance range of the output path.
The locations at which the light beams are incident on the multiplexer will be referred to as input ports. To ensure that each light beam incident on the optical multiplexer enters the output path after passing through the optical multiplexer, the light beam has to be accurately aligned relative to the input port both spatially and angularly such that the output beam output by the optical multiplexer is incident on the output path within the acceptance range of the output path. Each input port can be regarded as being characterized by an acceptance range. Light incident on the input port and output by the optical multiplexer such that the light is incident on the output path within the acceptance range of the output path is said to be within the acceptance range of the input port. The acceptance range of the input port has a spatial component and an angular component that interact such that the spatial component is at a maximum when the angular component is at a minimum and vice versa.
In a multiplexer without adjustments, such as the monolithic multiplexers shown in FIGS. 6A-6D and 7A-7D of the above-mentioned patent application, the acceptance range of the input ports has a fixed relationship to the acceptance range of the output path, although this relationship differs from example to example, depending on the orientation tolerance of the constituent beam splitting surfaces. In such multiplexers, the source of each light beam has to be adjusted spatially and angularly to align the light beam incident on the input port so that the light beam is incident within the acceptance range of the input port. The source of the light beam is typically a laser, such as a gas laser or a semiconductor laser. Lasers, especially gas lasers, are typically bulky and heavy, which makes the adjustment process difficult. Moreover, if a light source fails in the field, the replacement light source has to be adjusted to align the light beam within the acceptance range of the input port.
In a non-monolithic multiplexer, such as the non-monolithic multiplexers shown in FIGS. 5A-5D of the above-mentioned patent application, an ability to adjust the components of the multiplexer spatially and angularly provides a variable relationship between the acceptance ranges of the input ports and the acceptance range of the output path. Consequently, such multiplexers are adjustable such their input ports have can have a substantially greater acceptance range than the input ports of a monolithic multiplexer. However, non-monolithic multiplexers are substantially larger than monolithic multiplexers, so that the output path is separated from the input port of the multiplexer by a larger distance. The increased distance makes the adjustments more critical. Moreover, the process of adjusting the components to compensate for spatial and angular differences between the light beam and the nominal location and direction of the input port is very complex because the adjustments interact. Adjustment times measured in hours are typical. Moreover, users of adjustable multiplexers on encountering a problem will often adjust the multiplexer in an attempt to correct the immediate problem for a particular wavelength or application. Such adjustment may well put the multiplexer further out of alignment for other wavelengths or applications, correction of which will then require a complete re-adjustment of the multiplexer.
In many applications, the output path to which the multiplexer outputs light includes a converging lens whose focusing effect substantially increases the spatial acceptance range of each input port. With such a lens, the acceptance range is approximately equal to the spatial range over which the lens has a spherical aberration less than the diameter of the optical waveguide. However, a lens does not provide a corresponding increase in angular acceptance range. Accordingly, conventionally, a highly-precise angular adjustment of either or both of the light source and the multiplexer is needed to ensure that the multiplexer outputs the light beam incident on each input port to the output path.
Accordingly, what is needed is a multiplexer that can be used without the need to perform highly precise spatial and angular adjustments of the light sources or without the need to perform precise and interactive adjustments of internal components of the multiplexer.